Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios

[Vernon Yang]

On Sun, Feb 8, 2026 at 6:25 AM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
>
> On 2/7/26 23:17, Barry Song wrote:
> > On Sun, Feb 8, 2026 at 6:05 AM David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> >>
> >> On 2/7/26 23:01, Barry Song wrote:
> >>>
> >>> Maybe change “just not skip” to “just skip”?
> >>>
> >>> If the goal is to avoid the collapse overhead for folios that are
> >>> about to be dropped, we might consider skipping collapse for the
> >>> entire VMA？
> >> If there is no memory pressure in the system, why wouldn't you just want
> >> to collapse in a VM_DROPPABLE region?
> >>
> >> "about to be dropped" only applies once there is actual memory pressure.
> >> If not, these pages stick around forever.
> >
> >   agree. But this brings us back to the philosophy of the original patch.
> > If there is no memory pressure, lazyfree folios won’t be dropped, so
> > collapsing them might also be reasonable.
>
> It's about memory pressure in the future.
>
> >
> > Just collapsing fully lazyfree folios with VM_DROPPABLE while
> > skipping partially lazyfree VMAs seems a bit confusing to me :-)
>
> Think of it like this:
>
> All folios in VM_DROPPABLE are lazyfree. Collapsing maintains that
> property. So you can just collapse and memory pressure in the future
> will free it up.
>
> In contrast, collapsing in !VM_DROPPABLE does not maintain that
> property. The collapsed folio will not be lazyfree and memory pressure
> in the future will not be able to free it up.

Thank you Barry for pointing out this corner case, and thank you David for
suggestions and explanations.

LGTM, I will fix it in the next version.

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Thanks,
Vernon